Flexible couplings have long been used between a driving member and a driven member to transmit torque and to accommodate axial, torsional or parallel, and angular misalignment. Flexible couplings tend to smooth torque variations from a power plant and can reduce driveline vibration due to vibrational resonances and the like. Additionally, couplings which exhibit predetermined torque limits are known, such as Dynafiex.RTM. flexible couplings available from Lord Corporation of Erie, Pa., the assignee of the current invention.
U.S. Pat. No. 4,938,723 to Yoshimura et al., which is hereby incorporated by reference herein, describes a torque limited coupling similar to the LC-type of Lord Dynafiex coupling wherein a low torsional spring rate is provided by a shear elastomer section for in line vibration isolation, yet when a predetermined torque limit is exceeded, the coupling slips within the housing 105 to prevent over-stressing of the shear elastomer section.
FIG. 3 of the '723 patent describes a coupling similar to the Lord LC Dynaflex flexible coupling which includes a housing 105, an elastomer 107, and an inner member 108. The elastomer 107 is bonded to the inner member 108, but not to the housing 105. The elastomer 107 is interference fitted within the housing 105 such that when the predetermined torque limit is exceeded, the elastomer 107 slips within the housing 105, thus, protecting the elastomer from being over-stressed. For some applications this coupling is inadequate because it only exhibits a single spring rate within the couplings operating range, i.e., before the torque limit is reached.
U.S. Pat. No. 2,869,340 to Saberton describes a shaft assembly which exhibits dual-rate characteristics. The coupling assembly includes a elastomer sleeve 11 comprising a first stiffness element. Buffer blocks 14 supply a secondary spring rate when the torque is large enough to allow buffer 14 to contact radial members 13. This coupling has several disadvantages in that it requires multiple assembly steps to attach the buffers 14 and 15. Furthermore, the buffer exhibits highly non-linear characteristics, i.e., it stiffens greatly upon being loaded because the buffer 14 is loaded in compression loading.
U.S. Pat. No. 4,861,313 to Zeiser et al. describes an elastomeric shaft coupling for concentric shafts which includes compliant portions 20 and 50 which further include teeth 90 and 92 which interact to limit or snub motion after a certain torque or deflection is met. This snubbing interaction provides a rigid connection for high torque loading upon meeting the torque or deflection limit. Although this coupling has the ability to carry high torque loads, it lacks the ability to smooth out torque variations when transmitting high loads because the connection is essentially rigid when the teeth interact to cause snubbing.
U.S. Pat. No. 3,727,431 to Yokel describes a flexible torsional coupling including a resilient material 20, an outer ring OR including an abutment 27, and an inner ring IR including an abutment 25. A skin of elastomer covers the abutment 25 and a cushion 35 is attached to the abutment 27. Under large torsional loads, abutment 25 formed on inner ring IR will contact abutment 27 formed on outer ring OR. Rubber cushion 35 prevents shock loads on components upon initiation of snubbing. Although this cushion 35 does tend to smooth out the torque variations somewhat, it is ineffective because the cushion is highly non-linear because it is a compression-loaded element.
U.S. Pat. No. 4,467,753 to Lange describes a vibration damper for use on an engine shaft. FIG. 4 of the '753 patent describes using an annular elastomer layer 66 bonded between a metal pulley and a plate 67. This shear elastomer element is effective in reducing shock loads during startup. However, it is only a single-rate device and does not include any means for protecting the elastomer from becoming over-stressed.
U.S. Pat. No. 4,257,242 to Domer et al. describes a resilient coupling including a first elastomer block 7, elastomer joints 10 and 11, armature 1.sub.1, and a core 8. This coupling provides an inner shear element 10 and 11. However, the second element 7 is a compression loaded element, and thus, is highly nonlinear. Furthermore, the only means for snubbing is for 1 and 1.sub.1 to contact which results in an undesirable abrupt, rigid, metal-to-metal connection.
For certain applications, such as in the driveline of a tractor, a flexible coupling is desirable for attaching between the engine flywheel and the transmission. It is desired that the coupling helps to smooth out torque variations and to reduce the resonant frequency of the shaft system. Without a coupling, the shaft resonance may, and usually does, lie within the operating range; therefore, a soft flexible coupling is desired to reduce the shaft resonance to a frequency well below the normal operating frequency range. However, this may introduce a secondary problem of large transient motions during startup and shutdown conditions due to the soft spring rate. These large motions can exceed the backlash of the gear train or transmission and create a noisy gear rattle which could possibly be a mechanically detrimental condition.
To solve this problem, it is thought desirable to include a dual-rate coupling. However, all of the prior art couplings are inadequate for this application because they lack the combination of functional features required. All of these devices described above lack the means for snubbing one section while allowing the other section to operate freely. Furthermore, the prior dual-rate couplings exhibit highly non-linear characteristics because at least one of the elastomer sections is loaded in pure compression.